BTD, it's not a new idea, just wanted to clear that up.
But it's all that is required. See you can dry with epsom salts straight out of the box (still in this case a teaspoon may dehydrate 4ltrs, really) but it might take a bit more.
Also if you don't use a relatively dehydrated form, then you might get a TINY sulfate impurity.
Because if you use a hydrated form it will dissolve, and if you use anhydrous or a hepatahydrate it will remain a solid, but the solid will become a more hydrated form of the solid.
But in any case it isnt going to cause you any problems.
One other thing swims noticed about epsom salts is that you can dry it, leave it uncovered in the box and it will still be hard and brittle a week later. Not technically anhydrous, as in that whole mass consists of anhydrous MgSO4..but that it contains a lot of the dehydrated forms, the wetter near the surface of the mass.
I remember stressing about the same things. First it was 'oh ive gotta filter through epsom salts...oh and its been exposed to the atmosphere I should add some as well'
It's add dryish epsom, cover it or screw on a lid for good practice, if any solids remain by the time its poured or used, it's deadly dry.
anyway thats the end of that.
hehe gonzo!